On Sunday, as Queen Elizabeth II and members of the extended royal family cruise through London as part of the Thames Diamond Jubilee Pageant, security forces will have their eyes on Her Majesty. Given the menace of terrorism, they’ll also be watching the water, the crowds and the sky. Explosives set on bridges could detonate as boats pass underneath. Snipers lurking in tall buildings could fire into the throngs of well wishers along the 11-km route. Hijacked airplanes could crash directly into the river. “A security coordinator is appointed to look at such issues, to take necessary precautions and to advise those in charge,” says Dai Davies, the former head of Scotland Yard’s Royal Protection Unit. “But the truth is that for a journey of this kind and this length, it’s impossible to have total security.”

With the Queen and around 50 members of the extended royal family all gathering in one place, the stakes are higher than ever. As such the London police, Scotland Yard and MI5 — Britain’s counterintelligence unit — have been considering a slew of ugly scenarios as they prepare for this weekend’s Diamond Jubilee, the biggest royal-security operation ever staged in the U.K. Agencies have digitally mapped key locations from the air to identify weak spots, like rooftops, and intelligence units have tuned in to phone conversations and monitored the Internet for chatter of potential Diamond Jubilee terror. On the day itself, more than 6,000 officers will line the route — some dressed in plain clothes and embedded with the crowds. And another 7,000 stewards will help with general crowd control.

And while it’s important to monitor those watching the pageant, police must also consider threats from within the pageant itself. Officers will search every person — and the more than 1,000 boats they’ll be sailing on — in the hours leading up to the extravaganza. At the same time, police frogmen will conduct underwater searches while other units scour riverbanks. The Royal Navy has confirmed that 10 of its vessels, some carrying commandos, will surround the royal barge carrying the Queen, Prince Philip, Prince Charles, the Duchess of Cornwall and the Duke and Duchess of Cambridge. The London police will also utilize 23 of its own boats and two borrowed from the nearby Essex police force. Whether discreet and hidden or oversize and flashy, many boats will contain specialist-firearm officers. But guns aren’t necessary to take someone out. As Davies says: “The Royal Marines are on fast vessels that can quickly intercept and, if necessary, ram and sink, anyone who tries to come too close.”

As in any high-level-security event, authorities have identified and isolated key groups that pose the biggest threat. Al-Qaeda is an obvious one. The group has developed expertise launching waterborne attacks in recent years. And then there are Irish republican militants. In 1979, the IRA assassinated Lord Mountbatten, the Queen’s cousin and close friend of Prince Charles, by detonating a 23-kg bomb attached to his fishing boat. In 1983, authorities foiled an IRA plot to blow up a bomb inside London’s Dominion Theatre while Prince Charles and Diana attended a Duran Duran concert. According to Davies, who is not directly involved in security operations for the Diamond Jubilee, the threat of a mainland attack remains low. Given recent events, though, security forces will still be wary. On May 18, police charged six men and one woman in Northern Ireland with a series of terrorist offenses including conspiracy to murder. Three of those charged are suspected members of the Real IRA, a splinter, militant Irish Republican Army faction. The arrests come just weeks before the Queen visits Northern Ireland on June 26 and 27 as part of her Diamond Jubilee celebrations. Authorities have not linked those charged with any threat to the Queen or the celebrations.

Not all threats operate within a network. Less known to the general public are “fixated individuals” — mentally ill people with a pathological focus on someone, often a stranger. Forty percent of the most persistent stalkers in the U.K. are fixated on members of the royal family, who collectively receive around 10,000 correspondences from them each year. Some believe they should have married Prince William. Others want to harm Catherine, Duchess of Cambridge. As one woman wrote on William’s Facebook wall ahead of his nuptials last year: “I will put a stop to this right now.” The Fixated Threat Assessment Centre — a team of psychologists funded by the Home Office and Britain’s National Health Service — identifies those who pose the biggest threats. “Ninety-eight percent of them are harmless,” says Davies. “But the younger, more attractive members of the royal family attract certain types of individuals who might try to do something for publicity.”

Naturally, protesters want attention too. Republic, a group that campaigns to abolish the British monarchy, has promised to stage “the biggest and boldest antimonarchy protest in modern times.” They’ve set up the website www.jubileeprotest.org.uk to keep their sansculottes informed. The main fear isn’t that they’ll actively harm anyone — Republic is a peaceful action group — but rather that the large crowds may trample people who, out of drunkenness or exhaustion, end up on the ground. A second fear is that members swept up in the spirit of dissent will harm themselves. “We remind people that their right to protest will be balanced against the rights of those people who have come to attend this historic event,” deputy assistant commissioner of the London police Stephen Kavanagh told the Evening Standard. “I would say that any individual who is thinking of making a protest and diving into the water should think very carefully about their own safety with the number of boats afloat.”

Members of the royal family will look after themselves too. Each of them — except for recent additions like the Duchess of Cambridge and the Duchess of Cornwall — have grown up with security guards at their side. They know better than anyone the numerous ways in which they could be harmed or even killed, and they find strength in the routines and protocols drummed into them from birth. That requires a certain leap of faith. “The Queen is very fatalistic. She trusts in the police and the others to do their jobs,” says Davies, adding that Elizabeth has had an easier time than her predecessors. “Queen Victoria was attacked eight times during her reign. This Queen has been relatively lucky.” In Elizabeth II’s 60th year on the throne, long may her luck continue.